The present disclosure relates to imaging of subsurface structures and hydrocarbon deposits. Specifically, the present disclosure relates to producing a seismic response in subsurface formations by applying a time-varying electromagnetic field into the subsurface structures in the presence of a static or time-varying magnetic field, and detecting the seismic response.
Seismic surveys are commonly used in hydrocarbon exploration. Processed seismic surveys provide information about structures of geologic layers and, in some cases, can predict fluid properties within the geologic layers. However, in seismic surveys, it is difficult to distinguish a formation containing hydrocarbon from a formation containing brine since seismic velocities from these two formations may differ by only a few percent. Assessments of hydrocarbon content from seismic surveys thus need to be corroborated by incorporating other geophysical data.
A distinguishable physical property between hydrocarbon and brine is resistivity/conductivity. It is to be noted that the terms resistivity and conductivity may be used interchangeably herein, and it is understood that one of these terms is the inverse of the other. Obtaining an accurate map of subsurface conductivity thus can be advantageous. A reservoir rock filled with brine when compared to a reservoir rock filled with hydrocarbons may exhibit a contrast of 10-100 times in conductivity. In other words, subsurface conductivity can be particularly useful in distinguishing a brine-saturated formation from a hydrocarbon-saturated formation.
One conventional technique used for obtaining subsurface resistivity maps is controlled-source electromagnetics (CSEM). To detect the presence of hydrocarbon at a presumed depth, CSEM typically employs a current dipole as a source. The dipole transmits an electromagnetic field into the subsurface, and receivers at the surface are used to measure electromagnetic fields reflected by subsurface formations. The vast majority of CSEM surveys are done in the frequency domain where amplitudes and phases of the reflected electromagnetic fields are recorded. For these frequency-domain surveys, the difference between electromagnetic fields from subsurface formations with and without hydrocarbon is most distinguishable when the receivers are placed at a far offset from the source relative to the presumed depth of the hydrocarbon, such that direct coupling between the source and the receivers is minimized.
However, images produced through a model-based inversion process using CSEM data recorded by the receivers exhibit poor spatial resolution. The poor spatial resolution is due to the fact that electromagnetic radiation propagates diffusively in the subsurface, and even more so when the frequency of the electromagnetic field from the source is low. In CSEM, the frequency of the transmitted electromagnetic field is typically kept low (e.g., in the range of 0.1-5 Hz) because there is a practical upper limit on the frequency. This upper limit results from the fact that time-varying electromagnetic fields tend to decrease exponentially in subsurface formations due to attenuation. Diffusive propagation occurs because the Earth has a modest electrical conductivity giving rise to a skin effect phenomenon. The electric fields tend to decrease faster as the conductivities of subsurface formations increase and as the frequency of the electromagnetic field from the source increases. Therefore, for deeper targets, there is a limit to the achievable spatial resolution of the resistivity maps.
Moreover, in CSEM, the inversion process is inherently ill-posed, or under-constrained, and thus requires heavy regularization and/or substantial prior information and assumptions in order to converge to a solution. Consequently, the inversion process is susceptible to producing inaccurate resistivity maps since different subsurface models may yield the same measurements by the receivers.
There are other methods that also measure the electrical response of the subsurface to generate resistivity maps. These methods usually differ by the acquisition geometry, specifically, by the placement of the source and receivers. Some of them use a vertical dipole source, some detect time-domain signals, while others measure field gradients instead of the fields. However, all of these methods still suffer from the same limitations associated with skin depth and the diffusive nature of the electromagnetic fields in the subsurface, as mentioned above.
Another technique known as electro-seismics—sometimes referred to as electrokinetics—relies on electromagnetic energy to seismic energy conversion resulting from electrokinetically induced fluid movement at interfaces between subsurface formations. Electro-seismics uses a source to transmit an electromagnetic field into the subsurface and receivers at the surface to measure seismic signals, or waves, generated from the conversion. Seismic response from a layer in the Earth comes from the electric field perpendicular to a layer and the change in the electrokinetic coupling coefficient between the layer and its adjacent layers. The response is subject to tuning effects where electrokinetic conversion from the top and bottom of the layer produces a seismic interference. This electro-seismics technique can, in some instances, produce high resolution images using seismic imaging types of algorithms and does not have to rely on an inversion method. However, the measured seismic signals depend on properties of the subsurface formations, such as electrokinetic coupling coefficients and permeabilities, which are usually poorly constrained. Consequently, it is often difficult to correctly interpret the measured seismic signals because they are not necessarily proportional to the thickness of the resistive anomaly.
In light of the above described drawbacks of conventional technologies, the inventors recognized a need in the art to develop alternative methods to generate subsurface conductivity maps that overcome or improve upon such limitations.